A pump of this type comprises a tank having an inlet for sewage water and one or several electrically driven pumps arranged in the lower part of the tank. The pump/pumps are connected to a pressure pipe which brings the pumped water away.
When using pumps it is of course important to avoid dry operation as much as possible as this increases wear and energy demand. It is also an advantage to let the surrounding water cool the electric motor which means that the pumps are normally stopped when the water level has reached the upper part of the pump.
The impulses to start and stop the pump/pumps may be obtained by help of level switches which are arranged at different levels in the tank or by means which monitor the current consumption. There are different systems for this which include possibilities to alternate the pumps in a tank containing at least two pumps and also to initiate an earlier or later start if the previous operation cycle has been long or short caused by a large or a small inflow to the tank. Compare the Swedish Patents No:s 469 408 and 420 788.
A disadvantage with an automatic stop of the pump when the water level has reached the upper part of the pump is that sludge and other pollutions easily collect within the lower volume of the tank which is never emptied. These pollutions easily stick to the pump impellers and might mean very frequent service intervals. The present development towards narrow pump stations increases the problems. A common way to solve an acute situation is to operate the pumps backwards by disconnecting the automatic control.